Many high speed data transmission networks rely on optical links to communicate digital data. For each direction of such an optical link, the link includes an optical transmitter (such as a laser), an optical fiber, and an optical receiver (such as a photodiode). The optical transmitter converts outgoing electrical data signals into outgoing optical data signals. The optical receiver converts incoming optical data signals into incoming electrical signals. Optical transmitters are also sometimes called “optical sources” and optical receivers are also sometimes called “optical detectors”.
Optical transmitters and receivers are often manufactured on a semiconductor substrate using semiconductor processing technology, which uses a recipe of processing steps to create an appropriate structure having very fine feature dimension sizes. Processing technology results in some imperfections. Sometimes, such imperfections may result in a dysfunctional or unreliable structure. The ratio of successfully fabricated structures to the total number of structures attempted is often termed “yield”. All things being equal, higher yields are more desirable. However, higher yields often require undesired tradeoffs with performance or operating conditions or at the least more design or process development which can take a great deal of time and expense. Thus, the most commercially viable yield is often less than the maximum possible yield.
As an example, suppose that the structure being manufactured is an optical transmitter in the form of a laser, and that nine out of ten of such structures are manufactured successfully. The laser could then be said to have a yield of 90 percent, which may very well be an acceptable yield for many applications.
Sometimes, however, it is desirable to have an array of optical transmitters as a final integrated product. For instance, suppose that one desires a cable with 10 separate bi-directional optical links. It might be helpful to have a single integrated die with 10 optical transmitters built thereon. Such an array might be more compact, and be more convenient to align. However, the yield of the overall die can plummet in such an arrangement. For instance, if a successful structure relies on 10 working optical transmitters, and each optical transmitter can be manufactured with a 90 percent yield, the yield of the entire array would be much smaller, even as low as 0.9 to the power of 10 (expressed sometimes as 0.910, which is about 35%) in situations in which the yield of one optical transmitter was independent of another. This yield can be improved by tightening operating conditions or relaxing specifications, but this may be unacceptable or undesirable for the intended application.
What may be even more problematic than poor initial yield is poor device reliability. Unacceptable reliability is a common problem with semiconductor lasers. Such problems can typically be divided into those with high initial rates (often termed “infant mortality”), and those with much longer mean times to failures but which still result in unacceptable long term reliability. Once identified and characterized, infant mortality failures are typically screened out by operating the device (at the wafer, die, optical subassembly or final transmitter or transceiver module level), for a predetermined period of time, and usually at an elevated operating temperature and/or current density.
Failure modes with much longer mean times to failure can be avoided by testing representative device lots, usually over a range of elevated temperatures and current densities to confidently predict acceptably low failure rates over specified operating conditions or to drive design changes to achieve these failure rates.
As with initial failures, reliability problems can be greatly exacerbated in arrays of devices, and such problems have continued to be a major design and cost issue in parallel optical links.
One solution to improve optoelectronic array device reliability is the use of a multiplicity of optical sources all coupled to the same optical channel. This may be done at good coupling efficiencies if the optical channel has greater modal volume (e.g., supports a larger number of electromagnetic modes) than the combined modal volume of the sources. This is often the case with common VCSEL dimensions and multimode optical fiber designs. In this solution, a single source is used to transmit a desired signal. However, if a failure or degradation of this source is detected, a different source of the subarray is coupled to the same optical channel.
There are several drawbacks to the above-mentioned technique. For instance, the number of sources is the product of the array size and the number of devices in the subarray (4 per channel in the prior art). Such a large number of sources uses a much larger number of connections to the array of driving circuits. This might be only practical with the use of direct connection (e.g. flip chip binding) of the optoelectronic array to the driver array wafer. Additionally the smaller devices needed may have lower yields or reduced reliability compared to the larger single device it replaces. Finally, many yield and reliability problems are due to local material defect densities and result in much less improvement in overall yield of a small group of devices relative to single devices than would be expected in independent failure mechanisms.